Consolidate CI providers about hunter

[cristianadam]

I would suggest using GitHub Actions, see #40

Virtual environment	YAML workflow label
Windows Server 2019	windows-latest
Ubuntu 18.04	ubuntu-latest or ubuntu-18.04
Ubuntu 16.04	ubuntu-16.04
macOS Catalina 10.15	macos-latest

Each virtual machine has the same hardware resources available:

• 2-core CPU
• 7 GB of RAM memory
• 14 GB of SSD disk space

Each job in a workflow can run for up to 6 hours of execution time.

I've blogged about how to use C++ and CMake with GitHub Actions here: https://cristianadam.eu/20191222/using-github-actions-with-c-plus-plus-and-cmake/

[hjmallon]

Apvveyor is hellish slow too, Travis will run many configurations (around 5?) at once, but Appveyor only runs 1. Boost test builds take ~5hrs for me on Appveyor, but far less on Travis.

[rbsheth]

Closed with #250, everything is on Github Actions now.